Particular embodiments generally relate to dimming of electronic lamps.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
A dimmer, which includes a triode for alternating current (TRIAC), is used for dimming of incandescent lamps. The dimmer may use forward or reverse phase control. Both phase control schemes chop an alternating current (AC) line voltage either at the beginning of the half sine waveform (forward phase control) or at the end of the half sine waveform (reverse phase control). This stops the power delivered to the incandescent lamp for an adjustable/controllable part of the sine waveform, which is referred to as a non-conduction angle. The conduction angle is the part of the sine waveform where power is delivered. The ratio between the conduction portion and the full waveform defines the dimming level.
The above type of dimming uses the characteristics of the TRIAC. For example, the TRIAC can be turned on at a controlled moment and after that, the TRIAC stays in full conduction until the current through the TRIAC goes under a sustaining level in either direction. For example, when the sine waveform crosses zero, the current goes below the sustaining level and the TRIAC is turned off.
FIG. 1 depicts an example of a dimming circuit 100. A phase control circuit 106 is used to trigger a DIAC 105 at a controlled moment, the DIAC 105 then turns on the TRIAC 104. To operate phase control circuit 106, a variable resistor R, and a capacitor C are mounted in series with an incandescent lamp 102. Incandescent lamp 102 acts as a resistive load and offers a continuous path to ground that allows current to flow through variable resistor R and capacitor C when TRIAC 104 is turned off. This allows a continuous flow of current that charges capacitor C in a desired amount of time that is set by variable resistor R. When capacitor C builds up a certain amount of charge and its voltage reaches the breakover voltage of DIAC 105, TRIAC 104 begins to conduct and turns on incandescent lamp 102. The amount of time is set based on the conduction angle that is desired. A dimmer switch knob or slider could be used to control the conduction angle.
When a light-emitting diode (LED) lamp is used instead of incandescent lamp 102, LED lamp is driven by an electronic circuit that mainly includes a power converter and control circuits. Issues result when the LED lamp is used with TRIAC 104, such as flicker, in-rush current, dead travel, pop-on, etc. These issues may result because TRIACs were designed to drive a resistive load, such as incandescent lamp 102, instead of an electronic load, such as an LED. When forward phase control is used, a big inrush of current occurs when conduction begins. This is because a voltage level suddenly increases from zero to a high level. FIG. 2 shows an example of a graph 200 showing a forward phase control waveform according to one embodiment. In a first section 202, TRIAC 104 is not conducting. At 204, TRIAC 104 begins conducting. At this point, the voltage goes from zero to a high level at 206. The shaded part indicates the time in which TRIAC 104 is conducting. The inrush current may create noise in the system and also a large oscillation that may lead to TRIAC 104 turning off improperly.
Another disadvantage with using TRIAC 104 is that dimming the LED lamp using the electronic circuit that drives the LEDs may be difficult using TRIAC 104 because TRIAC 104 needs a hold current as several milliamps (mA) to several tens of milliamps. When the current through TRIAC 104 is lower than the hold current, TRIAC 104 will shut down. Therefore, current to hold TRIAC 104 on when the LED goes into a deep dimming level is not enough, which makes it hard to control the LED lamp when it goes into a deep dimming condition. This may also cause a pop-on condition where the LED lamp is turned off under deep dimming level. The LED lamp cannot be turned on from that dimming level until setting the dimming level back to a high dimming level, which causes the LED lamp to suddenly pop on.
Also, the current waveform input into the LED lamp intrinsically has high harmonics when the voltage waveform is conducted as shown in FIG. 2. These high harmonics eventually make it back to a power system for the LED lamp and create issues for power transmission, such as high losses and noise pollution for other electronic devices in the LED lamp.